1. Field of the Invention
The present invention relates to a hydraulic brake pressure control system of the load responsive type for wheeled vehicles, and more particularly to a hydraulic brake pressure control system of the type which is arranged to control the rate of increase of the wheel brake cylinder pressure less than that of the master cylinder pressure in proportion to the vehicle load.
2. Description of the Prior Art
In Japanese Patent Publication No. 46-39963, there has been proposed a hydraulic brake pressure control system of the type which includes a modulator valve mounted on an unsprung mass of a vehicle, the modulator valve having a spring-loaded valve member carried on a load sensing mechansim. The load sensing mechanism comprises a swingable lever pivotally mounted at its intermediate portion to the housing of the modulator valve to bear the valve member thereon, a first tension coil spring connected at its upper end to an upper portion of the valve housing and at its lower end to a first end of the swingable lever to bias the lever upward, a second tension coil spring connected at its lower end to a second end of the swingable lever and at its upper end to a sprung mass of the vehicle to bias the lever against the biasing force of the first tension coil spring.
In such a control system as described above, the first tension coil spring acts to define a hydraulic pressure applied to the wheel brake cylinders under control of the modulator valve when subjected to deceleration of the vehicle in excess of a predetermined value in a maximum loaded condition, while the second tension coil spring acts to control the hydraulic pressure in dependence upon the vehicle load. If in the control system the second tension coil spring was disconnected by a rebound stone, the entire biasing force of the first tension coil spring would be applied to the spring-loaded valve member in the modulator valve assembly. This results in an unexpected increase of the hydraulic pressure applied to the wheel brake cylinders when subjected to the deceleration of the vehicle in excess of the predetermined value in the maximum loaded condition. As a result, the road wheels of the vehicle tend to be locked in an unloaded condition or light loaded condition of the vehicle.